We have experimented quite extensively with various versions of stoves for the production of biochar. The most promising in my opinion is the following: a slightly modified version of the everything nice stove from Giovanni Di Maur.
The stove is based on a 60L metal drum, but can as well be made from a 180L standard oil drum. Equip yourself with two drums of the same size, an angle grinder, the proper protection equipment, a few self-cutting screws and an battery drill, a big screw driver and off you go.
Turn one drum upside down. The bottom of this drum will become the top of the stove. The drum for the outer barrel of the stove should be closed using the original screw caps. With the angle grinder, cut the top of the stove (bottom of the drum) open. Make sure to only cut the outer layer. In this way you will be able to put the lid back on with perfect fit.
For the air intake in the bottom of the stove (former top of the drum) cut a number of slots (about 6 cm in length for the 60L drum and 8 cm for the 180L drum). We have chosen a Fibonacci number. For the 60L drum 34 slots match well. For the standard oil drum, 55 suites best. Make the slots at an angle of about 30 degree. In that way, the airflow tends to go in a spiral between the inner and the outer barrel. Then the flame on top will form a nice spiral too. Bend the slots open with a large screw driver.
The radius of the inner barrel has to be 4 cm (5 cm for the large version) shorter than the outer barrel. To achieve this from the same type of barrel, cut both top and bottom off. I recommend to start taking one side off in the same manner as done with the top of the outer barrel. This way you make sure, to have a straight cut and the inner barrel stands firmly on the bottom of the outer barrel, which is important to avoid air coming into the inner char chamber. The other end should be cut to leave the inner barrel 5 cm shorter than the outer barrel.
Then take the drum without top and bottom and cut it open lengthwise. Overlap the cut edges and – with the help of 2 straps – tighten the barrel circumference until the barrel adopts the desired diameter. Fix the inner drum diameter by drilling a number of self cutting screws. Alternatively drill some holes and use bolts.
At about 8 cm from the bottom of the inner barrel, cut with an angle grinder a number of slots. Here the gases will diffuse out from the char chamber into the space between the barrels. According to my experience, the size of the slots is not so critical, but the collective pass should not exceed ¼ of the collective pass of the air intake in the bottom of the outer barrel. Here on the picture, the slots are much smaller, but can still be bent open.
Finally, make a hole in the lid of the stove. Ideally the hole should be round, which is not easy to achieve with simple tools. An octagon works well. And is a Fibonacci number too. The diameter of the hole in the lid should be half the diameter of the inner barrel. Watching through the opening, you can see the upper edge of the inner tube. When the stove is running properly, the flame will stand over this edge all around the inner tube.
It is very convenient to have an additional tube on hand, that can be put on top of the stove to regulate and increase the draft through the stove in case smoke develops.
Next time I build this type of stove, I will cut the outer barrel open both on top and bottom. Then I will tack weld the inner barrel onto the bottom plate. This will make sure that the inner barrel remains in the center position, even if you move the stove under process. This will be very helpful, because smoke develops sometimes later in the process, and tilting the stove usually increases the airflow and solves the smoke problem.
It is very important to use dry wood or biomass for biochar production. Start the process with a start fire on top of the filled inner chamber. When the stove runs properly, the wood gases come out from the slots of the inner barrel, so the flame runs as a spiral upwards in the outer chamber. No air can directly flow into the inner chamber. This is a very convenient system, as the stove can be fed with wood during the process through the hole in the top and you can watch the stuff becoming charred.